In recent years, reinforcing fiber composite materials obtained using reinforcing fibers such as carbon or aramid fibers have been utilized in structural materials for aircrafts or automobiles, sport and general industrial applications such as tennis rackets, golf shafts, and fishing rods, etc., by use of their high specific strength and specific modulus. Methods using a prepreg, which is an intermediate base in a sheet form comprising a reinforcing fiber impregnated with an uncured thermosetting or energy ray-curable matrix resin, are often used as methods for producing these fiber-reinforced composite materials. An epoxy resin is often used as the matrix resin used in this prepreg in terms of processability and handleability.
The matrix resin composed of the epoxy resin exhibits excellent heat resistance and favorable mechanical properties. However, a composite material prepared from this matrix resin sometimes has low impact resistance due to lower elongation and/or toughness of the epoxy resin than those of a thermoplastic resin. Therefore, improvement in these properties has been demanded.
For example, methods which involve formulating a rubber or thermoplastic resin having excellent toughness have heretofore been attempted as methods for improving the toughness of an epoxy resin. For example, studies on the formulation of a rubber such as an acrylonitrile-butadiene rubber having a carboxyl terminus into an epoxy resin have been conducted since the 1970s and well known in general. However, such a rubber has much lower physical properties such as a modulus of elasticity and a glass transition temperature than those of an epoxy resin. Therefore, the formulation of the rubber exhibits a reduced modulus of elasticity or glass transition temperature in the epoxy resin. Thus, it was difficult to keep a balance between the improvement in toughness and the modulus of elasticity or glass transition temperature. In another case, a particulate rubber such as a core-shell rubber is used for improving this disadvantage. However, the formulation of such a rubber in an amount increased for sufficiently improving toughness may exhibit a reduced modulus of elasticity or glass transition temperature in the epoxy resin.
Moreover, according to a known method for formulating a thermoplastic resin into an epoxy resin (see e.g., Patent Document 1), toughness is improved without impairing the mechanical properties of an epoxy resin by dissolving a thermoplastic resin such as polyether sulfone, polysulfone, and polyetherimide in the epoxy resin or formulating a fine powder of the thermoplastic resin into the epoxy resin to thereby uniformly disperse the thermoplastic resin in the epoxy resin. However, this method requires formulating the thermoplastic resin in large amounts. The formulation of the thermoplastic resin in large amounts may significantly increase the viscosity of the epoxy resin composition, resulting in a processability or handleability problem.
In recent years, studies have been conducted on improvement in toughness or impact resistance using a block copolymer composed of a diblock or triblock. For example, a method for improving toughness using a styrene-butadiene-methacrylic acid or butadiene-methacrylic acid copolymer has been proposed (see e.g., Patent Document 2). In this method, the effect of improving toughness has been confirmed in a combination using a bisphenol A-type epoxy resin as an epoxy resin which is in a liquid state at room temperature and 4,4′-methylenebis(3-chloro-2,6-diethylaniline) as a curing agent. Moreover, the reduction of heat resistance is kept as low as a few to a dozen ° C. However, the effect of improving toughness is still insufficient.    Patent Document 1: JP Patent Publication (Kokoku) No. 6-43508B (1994)    Patent Document 2: JP Patent Publication (Kohyo) No. 2003-535181A